Rare earth oxide superconductors and their ability to superconduct at significantly higher temperatures than previously recorded was first reported by J. G. Bednorz and R. A. Muller in 1986 in regard to mixtures of lanthanum, barium, copper and oxygen in an article entitled “Possible High Tc Superconductivity in the Ba—La—Cu—O system.” (64 Z. Phys. B.—Condensed Matter, pp 189-193 (1986)). Bednorz and Muller described Ba—La—Cu—O compositions that offered a substantial increase in the critical temperature at which the material becomes superconducting over what had been previously known for other classes of materials. Here, the composition was La5−xBaxCu5O5(3−y) where x=0.75-1, y>0, and the abrupt change in resistivity occurred in the 30 Kelvin range.
This contribution led to intensive investigation in order to develop materials having even higher transition temperatures, preferably above 77 Kelvin as this enabled the use of liquid nitrogen to cool the superconducting equipment. In 1987, C. W. Chu and co-workers at the University of Houston found that the onset Tc of the La—Ba—Cu—O compound could by increased to over 50 K by the application of pressure. (Phys. Rev. Lett. 58. 405 (1987); Science 235, 567 (1987)).
Chu and coworkers at Houston and at the University of Alabama subsequently discovered a mixed-phase Y—Ba—Cu—O system onset having Tc values near 90 K and a zero-resistance state at ˜70 K. This compound had the nominal composition Y1.2Ba0.8CuO4−δ. (Phys. Rev. Lett. 58, 908 (1987). Chu and coworkers as well as scientists at AT&T and IBM later showed this compound to consist of two phases of nominal composition Y2BaCuO5 (the “green” phase) and YBa2Cu3O6+x (the “black” phase). The latter phase was determined to be the superconducting phase, whereas the former was semiconducting (Cava et al., Phys. Rev. Lett. 58, 1676 (1987); Hazen et al., Phys. Rev. B 35, 7238 (1987); Grant et al., Phys. Rev. Lett. 35, 7242 (1987).
Superconductivity near 90 K was also reported in a mixed-phase Lu—Ba—Cu—O compound by Moodenbaugh and coworkers (Phys. Rev. Lett. 58, 1885 (1987). Chu et al. also identified superconductivity above 90 K for compounds of the formula ABa2Cu3O6+x, where A=Y, La, Nd, Sm, Eu, Gd, Ho, Er, or Lu (Phys. Rev. Lett. 58, 1891 (1987).
The data from these differing Rare Earth (RE)BCO (RE=rare earth, B=Ba, C=Cu) compounds demonstrated that for this class of compounds, the superconductivity is associated with the CuO2—Ba—CuO2—Ba—CuO2 plane assembly which can be disrupted by the A cations only along the c-axis.
Following this discovery, research was focused on the YBCO class of compounds with high temperature superconducting (HTS) properties. B. Batlogg first discovered and isolated the single crystallographic phase responsible for the superconducting properties of the YBCO compound. (B. Batlogg, U.S. Pat. No. 6,635,603). In isolating this single perovskite phase of a composition, Batlogg admonished that the composition was essential to isolation of the phase and that it must be within 10% of the M2M′Cu3O7−δ composition where M is a divalent cation preferably barium and M′ is a trivalent cation preferably yttrium.
Other studies have investigated both the effects of substitution of various rare earth elements for yttrium and of varying the 1:2:3 ratio of Y:Ba:Cu on the superconducting properties of HTS compositions. Multiple studies have shown the ability to partially or completely substitute rare earth elements except Pr, Ce and Tb and maintain a Tc of approximately 90 K for the resulting (RE)BCO composition. (S. Jin, Physica C 173, pp 75-79 (1991)). Additionally, further studies show that the c-axis coherence length and the Tc value increase with increasing ionic radius of the rare earth element substituted for yttrium (G. V. M. Williams, Physica C 258, pp 41-46 (1996)).
Building on these discoveries, P. Chaudhari and his co-workers at IBM developed a method for making thin films of high temperature superconducting oxides with a nominal composition of (RE)(AE)2Cu3O9−y where RE is a rare earth element, AE is an alkaline earth element and y is sufficient to satisfy valence demands. (Chaudhari, U.S. Pat. No. 5,863,869 (1999)). The rare earth elements used included Y, Sc and La, and AE could also be substituted for by Ba, Ca or Sr. Copper was the preferred transition metal for the oxide due to its high superconducting onset temperature and the smooth, uniform properties of the copper oxide films. Using this growth process, Chaudhari was able to obtain YBCO films with superconducting onset temperatures of about 97 Kelvin that exhibited superconducting behavior from 50 Kelvin to in excess of 77 Kelvin. These films were within 15% of the targeted (RE)(AE)2Cu3O9−y composition, and Chaudhari noted that the exact composition was not necessary in order to observe high temperature superconductivity.
However, in another study of (RE)BCO cation exchange in thin films, J MacManus-Driscoll et al. noted that Tc decreased dramatically for off-composition films with substitutions of rare earth (RE) elements on the Ba site such as RE(Ba2−xREx)Cu3Oy where RE=Er or Dy and x>0.1 (14% deviation) and where RE=Ho and x>0 (any deviation). (J. L MacManus-Driscoll, Physica C 232, pp 288-308 (1994). J. MacManus-Driscoll further reported that the oxygen pressure at which the thin films were grown seemed to have an effect on the structural disordering of the RE and Ba cations as did the rare earth ion size. Small rare earth cations substituting for the larger Ba cations would produce large strains on the lattice and therefore an unstable phase which would not likely occur.
Another study of varying the 1:2:3 stoichiometry of YBCO thin films noted that large excesses of yttrium formed ultra small yttrium precipitates leading to increased surface resistance (Rs) and poor microwave quality but that a slightly enhanced copper and yttrium content lead to minimum surface resistance (E. Waffenschmidt, J. Appl. Phys. 77 (1) pg 438-440). Furthermore, N. G. Chew et al. analyzed the effect of slight changes in composition on YBCO thin film structural and electrical properties and discovered that films grown with a stoichiometry close to 1:2:3 or with excess yttrium are smooth while films with excess barium exhibited surface roughness and growth of a-axis-oriented grains. (N. Chew, Appl. Phys. Lett. 57 (19) pp 2016-2018 (1990). These authors further found that there is a well defined YBCO composition where Tc and Jc are maximized and the c-axis lattice constant, (007) x-ray peak width, and surface roughness are minimized. These quantities were optimized for a Ba/Y ratio of 2.22±0.05 (subsequently suggested to instead be equal to 2) and a Cu/(Y+Ba+Cu) ratio of 0.5. Slight changes in cation ratios away from this optimized composition caused significant degradation in the parameters listed above.
W. Prusseit et al. have created an iso-structural Dy-BCO thin film material with improved properties compared to their YBCO films. By substituting dysprosium for yttrium and growing under identical conditions as YBCO, Prusseit created films that deviated only slightly from the 1:2:3 stoichiometry. Compared to their YBCO films, these materials exhibited better chemical stability and enhanced transition temperatures (by 2-3 K), and they also had a 20% reduction in surface resistance (Rs) at 77 K: ˜250 μΩ vs. ˜300 μΩ at 10 GHz, measured in a microwave cavity (W. Prusseit, Physica C 392-396, pp 1225-1228 (2003)). Hein (High-Temperature Superconductor Thin Films at Microwave Frequencies (Springer Tracts in Modern Physics, 155), Berlin, 1999) and others have measured somewhat lower surface resistance, ˜200 μΩ at 10 GHz and 77 K, in cavity measurements of YBCO thin films.
The compositions of these (RE)BCO compounds may be altered substantially from the nominal 1:2:3 stoichiometry in order to optimize their properties for specific applications. It is the primary object of this invention to provide high temperature superconducting thin films that have the lowest possible RF surface resistance (Rs) values as well as the lowest achievable RF nonlinearities. This often requires fabrication of (RE)BCO films that deviate significantly from the 1:2:3 composition. It is another object of this invention to provide a thin film superconductor that is optimized for RF/microwave applications. It is another object of this invention that the film has a low surface resistance. It is another object of this invention that the film has a highly linear RF/microwave surface reactance. It is another object of this invention that the stoichiometry of the film deviates by at least 10% from the standard 1:2:3 stoichiometry and with full substitution for yttrium by a rare earth element.